There are many known methods of retransmitting radio wave signals intra-building and inter-building as well as ground to aircraft or ground to satellite. Some techniques modulate the reflection coefficient of an antenna. Prior art designs have used frequency multiplication or field effect transistor (FET) mixing to modulate the reflection coefficient of an antenna and therefore the reflected amount of incident power to perform this function. This method is limited by its instantaneous bandwidth to about 50 MHz and its overall efficiency in addition to the tendency to produce large amounts of harmonics.
As shown in FIG. 1, in this method, a pin diode or FET 10 is connected to the transmission line port of an antenna 12. The reflectivity is controlled by a signal bias 14. The gate voltage changes the reflection coefficient of the drain, so a varying level of power is reflected from the antenna.
Other techniques include optical retroreflectors and RFID transponders. Optical retroreflectors reflect the illuminating energy. RFID transponders use a fixed RF frequency with resonant circuits.
U.S. Pat. Nos. 5,073,900 and 5,339,330, both entitled “INTEGRATED CELLULAR COMMUNICATIONS SYSTEM” disclose a cellular communications system having fully integrated surface and space nodes. A spread spectrum communications method is used to increase the number of users that are accommodated within the spectrum. High gain antennas are used to keep power requirements at the mobile handsets to a minimum, reducing the need for power at the transmitter, but not the local transmitter itself. The present invention is not limited in this regard. U.S. Pat. Nos. 5,073,900 and 5,339,330 are hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,603,098 entitled “INTEGRATED RADIATING AND COUPLING DEVICE FOR DUPLEX COMMUNICATIONS,” discloses a radiating and coupling device having an antenna, a circulator, a receiver section, an input/output device section, and a transmitter section. A transmitter section is required to retransmit the signal. The present invention is not limited in this regard. U.S. Pat. No. 5,603,098 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,819,164 entitled “MODULATED RETROREFLECTION SYSTEM FOR SECURE COMMUNICATION AND IDENTIFICATION,” discloses a transponder that receives, modulates, and reflects a signal back to a transceiver. The transceiver comprises a frequency source oscillator connected to a bi-directional antenna through a circulator, and a mixer detector. An interrogation signal is generated by the oscillator and transmitted by the antenna to the transponder. The transponder reflects the signal back, where it is received by the antenna and fed into the mixer detector. A local oscillator is required. The present invention is not limited in this regard. U.S. Pat. No. 5,819,164 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,456,668 entitled “QPSK MODULATED BACKSCATTER SYSTEM,” discloses a method of using quadrature phase shift keying to modulate radio signal. One antenna is used to receive a signal, which is amplified and modulated using a sub-carrier. The modulated signal is then retransmitted using a second antenna. A circulator could be used to separate the received signal from the retransmitted signal in applications where one antenna is used. The disclosed method requires a local transmitter to retransmit the modulated signal. The present invention is not limited in this regard. U.S. Pat. No. 6,456,668 is hereby incorporated by reference into the specification of the present invention.
There exists a need for a retroreflector that eliminates the power requirements of a local oscillator and does not suffer the limitations on switching speed of non-linear junctions that limit instantaneous bandwidth common in prior art designs.